In electronic color imaging, it is desirable to simultaneously capture image data in three color planes, usually red, green and blue. When the three color planes are combined, it is possible to create high-quality color images. Capturing these three sets of image data can be done in a number of ways. In electronic photography, this is sometimes accomplished by using a single two dimensional array of sensors that are covered by a pattern of red, green and blue filters. This type of sensor is known as a color filter array or CFA. Below is shown the red (R), green (G) and blue (B) pixels as are commonly arranged on a CFA sensor.
When a color image is captured using a CFA, it is necessary to interpolate the red, green and blue values so that there is an estimate of all three color values for each sensor location. Once the interpolation is done, each picture element, or pixel, has three color values and can be processed by a variety of known image processing techniques depending on the needs of the system. Some examples of the reasons for processing are to do image sharpening, color correction or half toning.
The diagram below shows how red green and blue pixels can be arranged in a particular color filter array pattern, hereinafter referred to as the Bayer color filter array. For a more detailed description see U.S. Pat. No. 3,971,065 issued Jul. 20, 1976 to Bayer.
G R B G
In commonly assigned U.S. Pat. No. 5,596,367 issued Jan. 21, 1997, incorporated by reference, adaptive methods of calculating green pixel values at pixels where green is not directly measured is described. These methods rely on algorithmically selecting one or two ways to calculate the missing green values. After determining the missing green values, the red minus green color difference values are computed at red pixel sites and the blue minus green color difference values are computed at blue pixel sites. The missing values for each color difference are computed by simple linear interpolation. Because the red, green, and blue CFA filters absorb two thirds of the incident light, such CFA sensors tend to be photometrically slow. Especially in low light situations, it would be better to use subtractive CFA filters such as cyan, magenta, yellow, and green. It would also be desirable to have a CFA pattern that permits adaptive interpolation of the luminance value (analogous to green) and permits for adaptive interpolation of both color difference values as well.